The present invention relates to a contact lens with a novel configuration of the posterior surface thereof, providing improved tear exchange and oxygen flow thereunder while the lens is being worn.
The invention also relates to a bifocal contact lens employing the simultaneous-vision principle and having a novel configuration of the posterior surface thereof.
The invention further relates to a bifocal contact lens employing the combined simultaneous alternating-vision principle.
Contact lenses are used primarily for correcting refractive aberrations of the eye such as astigmatism, myopia, presbyopia and others. The contact lens rests on the eye in contact with the cornea or the sclera, or both. The device serves as a new anterior refractive surface of the eye and retains some of the fluid between the cornea and the posterior surface of the lens.
Contact lenses presently known in the trade are divided into three main classes: soft lenses (hydrogels and silicon rubber), hard lenses (PMMA), and rigid gas permeable lenses (RGP). While contact lenses made of these materials each have their particular advantages and disadvantages, their common drawback is their inevitable interference with normal ocular metabolism and corneal health, primarily due to the greatly reduced oxygen supply and tear flow beneath the conventional contact lens which obviously lowers the rate of tear turnover which, in the uncovered cornea, is about 1.2 .mu.l/min. As the normal tear volume is about 6 .mu.l, the tear film, under average, normal conditions is replaced every five minutes. The tear film covering the cornea not only contains substances essential for lowering surface tension to maintain and enhance wettability of the epithelical surface of the cornea, but also supplies the oxygen vital for the metabolic processes sustaining the cornea, amongst them the enzymatic breaking down of glycogen, a high rate of which is essential for the normal functioning of the cornea. At the same time, tear flow is also instrumental in the flushing away of debris such as desquamated epithelial cells and various contaminants, most of which are products of the metabolic reactions.
In the absence of sufficient tear flow under a contact lens, the extended presence of the above debris and residues, and, first and foremost, oxygen deprivation or hypoxia, are liable to cause not only acute wearer discomfort, but severe clinical problems such as corneal abrasion, swelling, conjunctivitis and allergic reactions.
While the above-mentioned problems pertain to all contact lenses regardless of which material made, they are particularly pronounced with soft hydrogel lenses which are otherwise superior to hard and RGP lenses, particularly as to initial wearer comfort and easy fittability.
Although efforts have been made to improve contact lens performance with a view of remedy the above described shortcomings, none of the prior-art contact lenses have been known to give fully satisfactory service, in particular as far as extended-wear lenses are concerned.
While the above-mentioned problems are encountered with all types of contact lenses, mono- as well as bifocal, bifocal contact lenses of present design are known to suffer from an additional series of problems and limitations, such as poor distance or near vision, or both; poor wearer comfort: design features that run counter to the demands of ocular health: problems in locating the various bifocal segments and controlling position and orientation of the segment transition line and its degree of displacement relative to the pupil; presence of jump effect at the segment line and associated doubling of images; blurring at distance vision, and poor contrast sensitivity.